When it rains... it pours

A flurry of results from clinical trials of new HIV prevention strategies headline recent conference

By Andreas von Bubnoff and Alix Morris

Following on the heels of the STEP trial, which showed that Merck’s AIDS vaccine candidate (widely considered the most promising in clinical trials) was not effective, several other trials of novel HIV prevention methods have culminated. Unfortunately, many also ended with disappointing results.

The final data from two of these trials—one testing the effect of adult male circumcision on HIV transmission to women and another testing whether or not treatment of herpes simplex virus (HSV)-2, a common sexually-transmitted infection that causes genital warts, could reduce the risk of HIV infection—were presented at this year’s Conference on Retroviruses and Opportunistic Infections (CROI), held in Boston from February 3-6. Additionally, results from another circumcision study, which looked at the protective effect circumcision may offer against HSV-2 infection, were also presented.

Soon after, final results from the Phase III efficacy trial of a candidate microbicide gel known as Carraguard were released just prior to the opening of the Microbicides 2008 Meeting, which took place from February 24-27, in New Delhi, India. During the bi-annual conference, several updates on other trials and candidates were presented.

Extended benefits of circumcision

In heterosexual men, the protective effect of circumcision against HIV infection has been firmly established by three randomized, controlled clinical trials, showing that surgical removal of the foreskin reduces the risk of HIV acquisition by approximately 60% (see VAX January 2008, Global News). Additional studies presented at CROI looked at the effect of male circumcision on the acquisition of HSV-2, which is thought to increase the risk of HIV infection, and on transmission of HIV from infected men to their female partners.

Aaron Tobian of Johns Hopkins University reported results from a randomized trial conducted in Rakai, Uganda that enrolled over 3,500 uncircumcised men who were not infected with HIV or HSV-2. Half of them were randomly assigned to immediate circumcision, while the other half were offered circumcision at the conclusion of the trial. After two years, researchers observed that the risk of HSV-2 infection was reduced by almost 25% in circumcised men. “This might be part of the reason male circumcision decreases HIV acquisition,” says Tobian. Several observational studies have supported the role of HSV-2 infection in aiding HIV transmission (see VAX November 2005 Spotlightarticle, HIV prevention in a pill?). Infection with HSV-2 causes inflammation or, even worse, ulceration at the genitals, which is believed to make it easier for HIV to establish an infection.

While all of the male circumcision studies done so far were in HIV-uninfected men, Maria Wawer of Johns Hopkins University says that knowing the effects of circumcision in HIV-infected men is also important. After all the news about circumcision’s protective effects, Wawer says that in some communities, being uncircumcised may stigmatize men as being HIV infected. To avoid this, some HIV-infected men may seek circumcision.

This led Wawer and her colleagues to conduct another randomized trial in Rakai, Uganda that enrolled discordant couples—HIV-infected men with uninfected female partners. This allowed researchers to study both the safety and benefits of circumcision in HIV-infected men, as well as how circumcision impacts HIV transmission rates to female sexual partners. In this trial, 93 couples were enrolled in the intervention group, in which the male received immediate circumcision, and 68 couples were enrolled in the control group. Men in this second group were offered circumcision after the trial was complete.

After two years, researchers found that circumcision did offer some benefit to HIV-infected men—rates of genital ulcers were reduced by about 50% in circumcised trial participants compared to men in the control group. But circumcision had no effect on HIV transmission rates to the female partners of HIV-infected, circumcised participants. That result was “unexpected and disappointing,” Wawer says. “In previous observational data we had seen lower HIV rates in women married to HIV-positive circumcised men compared with HIV-positive uncircumcised men.”

Researchers suggest that one reason the female partners were not protected from HIV was because the couples resumed sex too soon after surgery. “If the males resume intercourse early after circumcision, before the wound is fully healed, there might be increased transmission,” says Wawer. Among 18 couples who reported resuming sex before the wound healed completely, 27% of the female partners were infected with HIV in the first six months of the study, compared to only 9.5% of female partners who became HIV infected after waiting to resume sexual activity. Wawer says it is very important that people not resume sex in the early postoperative period, even if they are not HIV infected. “In our trial of negative men the protective effects of circumcision became significant and apparent only after the six-month follow-up period,” she adds, referring to an earlier trial of male circumcision.

But even if circumcision does not offer a direct protective benefit to women, there are still benefits on the population level—if fewer men are HIV infected, overall infection rates in women will also be lower.

Stopping HSV doesn‘t stop HIV

More news on the HIV prevention front came from Connie Celum of the University of Washington in Seattle, who reported the results at CROI of a randomized clinical trial (HPTN 039) assessing whether or not administering an antiviral drug used to treat HSV-2 infection would decrease risk of HIV infection.

The trial enrolled 3,277 volunteers who were infected with HSV-2 but not HIV. Volunteers included men who have sex with men in the US and Peru, and heterosexual women at sites in Zimbabwe, Zambia, and South Africa. All participants randomly assigned to the intervention group received a twice-daily dose of an antiviral drug known as acyclovir that suppresses HSV-2. Volunteers in the control group received an inactive placebo. After 18 months, there was no difference in the number of new HIV infections between the two groups.

This came as a surprise to researchers. “Many people thought this would be a slam dunk,” says Celum. Although this was the first randomized, controlled trial to be conducted, several studies have suggested that HSV-2 infection increases susceptibility to HIV by two to three times, and therefore suppressing HSV-2 should have lowered the chance that volunteers would become HIV infected. The incidence of genital ulcers was reduced by 37% among volunteers who received acyclovir, but even this was much lower than what was observed in previous studies.

For now, it is still unclear why this trial led to such unanticipated results. “Why didn’t we have an effect on HIV at all?” Celum asks. She thinks it is unlikely that HSV-2 is not a risk factor for HIV, given the mounds of observed data that suggest otherwise, and instead suggests that adherence may be one factor that could have influenced the results. Volunteers in the study reported high levels of adherence—taking the medication as prescribed—but Celum said it could have been overestimated since it was all based on self-reported behavior. The less-than-expected reduction in occurrence of genital ulcers also varied geographically, suggesting biological reasons may also account for why researchers did not see any effect on HIV infection rates, says Celum. For example, there may be differences in how the drug was metabolized or in how susceptible HSV-2 was to the drug, based on the background characteristics of the population in which it was tested.

Microbicide results aren’t gelling

In February, the Population Council announced the results of a Phase III trial of Carraguard, a microbicide gel containing the compound carrageenan, which is a seaweed derivative used as a stabilizer and thickening agent in food and cosmetics. This randomized, double-blind, placebo-controlled trial was conducted at three sites in South Africa and involved 6,202 women between the ages of 16 and 72. Last year a Phase III trial with another candidate, known as cellulose sulfate, was terminated early by the trial’s data safety monitoring board (DSMB) because a higher number of HIV infections occurred among microbicide recipients than in those receiving a placebo gel (see VAX June 2007 Primer on Understanding Data Safety Monitoring Boards). Data collected in the Carraguard trial showed that 134 individuals who received the microbicide candidate became HIV infected, compared to 151 individuals who received an inactive placebo gel. The difference was not deemed statistically significant and researchers concluded that Carraguard was not effective at protecting women from HIV (see VAX February 2008 Primer on Understanding Biostatistics and the STEP Trial).

Measuring usage

Similarly to the HSV suppression study, a critical aspect of the Carraguard trial was women’s adherence to the product being tested. Women were counseled to apply the microbicide before every sex act, and although the self-reported adherence rates were 96%, researchers estimate that the actual adherence was much lower. To measure adherence to the gel, researchers collected behavioral information directly from participants and also conducted an applicator test. All applicators used to apply the gel were treated with a compound that, when subjected later to a stain, would change colors if it had been exposed to vaginal mucous. The results of these tests showed women used the gel in only 44% of sex acts, and only 10% of women were estimated to have used it during every sex act.

An applicator test is one method researchers are using to better estimate adherence, but even this approach is complicated. Barbara Friedland of the Population Council says it was difficult to determine the effectiveness of the applicator test. “All we can tell is whether the applicator was inserted in the vagina or not,” she says. “We don’t know when in relation to the sex act the applicator was inserted into the vagina.”

“It’s possible that low levels of adherence in the trial were responsible for why the product didn’t show an effect,” Friedland notes. “It’s also possible that there was a biological reason—it worked in the lab but it didn’t have the same effect in humans.” Researchers conducted tests of the microbicide in human cell cultures but did not conduct preclinical studies in nonhuman primates with the monkey equivalent of HIV, known as simian immunodeficiency virus (SIV), to gauge the efficacy of the product. Prior to initiating the Phase III trial, the Population Council conducted two Phase II safety studies of Carraguard in South Africa and Thailand, involving a total of 565 HIV-uninfected women.

Additional research was also presented at both CROI and Microbicides 2008 on the previously-halted cellulose sulfate microbicide trial conducted by CONRAD, a US-based reproductive health organization. After the trial was stopped by the DSMB, researchers tried to find out if the microbicide gel was in any way enhancing the risk of HIV infection. Researchers from Albert Einstein College of Medicine in New York conducted laboratory studies with the candidate microbicide in vaginal tissues. They found that cellulose sulfate disrupts the proteins that help form tight junctions between the cells that comprise the vaginal tissue layers, which are the first line of defense against HIV. This disruption makes it easier for HIV to cross the mucosal barrier (see VAX January 2008 Primer on Understanding HIV Transmission). These findings provide a possible explanation for how cellulose sulfate may have increased women’s vulnerability to HIV, and the researchers argue that this type of laboratory study should be conducted for all future microbicide candidates.

Together these results offer some sobering news for the HIV prevention field, but at the same time researchers also reported great progress in understanding basic scientific questions that open potential avenues of exploration for both HIV prevention and treatment.